The difficulty levels are assigned relative to the other material in the same genre or curriculum. So, comparing a rock song by Boston to a solo classical Bach piece is like comparing apples & oranges. All the rocks songs are in one category. Then, any new rock song or rock course, or technique tutorial is assigned a level relative to all the currently existing rock material. Same for the classical material. A new classical tutorial is assigned a difficulty level compared to the other classical material.
Hope this helps!
Keep rockin'
Mike
Keep rockin!
Mike Olekshy
GT Guitar Coach